Anhydro sugar alcohols, in particular derivatives of mannitol, iditol, and sorbitol, are known for their therapeutic uses and uses in food. At least one of these, isosorbide, 1,4:3,6-dianhydrosorbitol, is useful as a monomer in the manufacture of polymers and copolymers, especially polyester polymers and copolymers.
Anhydro sugar alcohols are produced by dehydration of the corresponding sugar alcohols (or monoanhydro sugar alcohols) by the action of various dehydration catalysts, typically strong acid catalysts.
Batch processes for the preparation of dianhydro sugar alcohols by acid dehydration are known in the art. For example, a batch process for the formation of the dianhydro sugar alcohol isosorbide has been described as a two-step process involving intramolecular dehydration of sorbitol to sorbitan (1,4-monoanhydrosorbitol), and further reaction of sorbitan to isosorbide (1,4:3,6-dianhydrosorbitol) in an acid-catalyzed dehydration-cyclization (R. Beck, Pharm. Mfg Inc. (1996), 97-100). Other monoanhydro by-products, 2,5-anhydro-L-iditol and 2,5-anhydro-D-mannitol, are also known to be produced under some reaction conditions (K. Bock et al., Acta. Chem. Scand. B 35, 441-449 (1981)).
For isosorbide to be used as a monomer in high volume polymers and copolymers for applications such as containers, it needs to be produced in large quantities, preferably in a continuous process.
International Patent Application WO 00/14081 describes a continuous process for producing anhydro sugar alcohols in which an organic solvent is used to dissolve the product and remove it from the reaction vessel.
U.S. Pat. No. 6,407,266 describes a continuous process for the dehydration of sugar alcohols. Purification of the crude reaction product may be accomplished by distillation, recrystallization, melt crystallization or a combination thereof.
The crude reaction product of prior art processes, as practiced using sorbitol, comprises about 70 to 80% by weight isosorbide and 20 to 30% byproducts. Several separation and purification processes for isosorbide have been disclosed in the prior art, including, for example, distillation, crystallization, chromatographic separation, and ion exchange. U.S. Pat. No. 4,564,692 discloses a process using crystallization from aqueous solutions to obtain the high purity needed for applications as polyol components in polyester and polyurethane polymers.
Commonly assigned U.S. Provisional Application No. 60/373,106 filed of even date herewith, discloses a combined reaction-separation process wherein dianhydro sugar alcohols are obtained as vapors in a stream of water vapor. The isosorbide condensed from such a vapor streams would require further purification to obtain the high level of purity needed for use in polymers such as polyesters, that is, at least 99.8% pure.
It is the object of the present invention to provide a novel, effective means of recovering and purifying dianhydro sugar alcohols from aqueous vapor streams, wherein purification by crystallization occurs while recovering the product by condensation, and a separate crystallization step is eliminated.